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The Walter Reed military medical center in Maryland was on lockdown on Tuesday, during an active shooter drill that many believed to be real.

The US Navy says a mistake using a mass notification system led to reports of an active shooter at Walter Reed National Military Medical Center.

A statement released Tuesday evening says that while preparing for an upcoming drill at Naval Support Activity Bethesda, the notification system was inadvertently activated without the words "exercise" or "drill."

According to the statement, people who saw the notification contacted security at NSA Bethesda, which launched an active shooter response.

Dutch Ruppersberger, a congressman for Maryland, told NBC News that he was at the site, which specialises in treating members of the military.

He said he was safe, and sheltering in a room with others.

I am currently at Walter Reed Medical in Bethesda where we've been told there is an active shooter. I am currently safe in a conference room w/ approx 40 others.